spaceprovided) Neurons in sensory areas of the brain receive continuously varying inputs in response to stimuli in the surrounding environment. In addition, they also receive spontaneous, intrinsically generated background input from the local network of cells. Despite numerous advances in neuroscience that have begun to elucidate sensory system function at the cellular level, we know surprisingly little about how individual brain cells integrate and process these two kinds of input. The experiments proposed here will examine the basic cellular rules governing the integration of visually driven synaptic inputs by single cortical neurons and explore how changes in network activity modulate synaptic input integration and cortical cells' input-output gain. The cellular mechanisms of input integration will be examined by recording intracellularly from primary visual cortex neurons in intact animals and analyzing the synaptic potentials and spike output evoked by visual stimulation. This work will provide insights into the fundamental cellular mechanisms underlying visual processing and elucidate basic principles of sensory cortical function.